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ELECTRICAL CIRCUITRY

  • US 3,710,149 A
  • Filed: 06/21/1971
  • Issued: 01/09/1973
  • Est. Priority Date: 06/21/1971
  • Status: Expired due to Term
First Claim
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1. In a combustion control system having a photoconductive photocell for sensing flame a regulated voltage source connected in shunt with said photocell and fail-safe circuitry responsive to changes in said photocell for energizing a relay when said flame is sensed, said fail-safe circuitry including at least one stage of amplification, the improvement comprising a fail-safe regulated voltage source monitoring means, said monitoring means comprising a source of standard voltage, a source of alternating voltage of predetermined magnitude, said monitoring means being arranged to operate in an operative mode when said regulated and standard voltages are within a predetermined range determined by the magnitude of the alternating current source, and a second inoperative mode when said regulated and standard voltages are outside said predetermined range, a solid state device having emitter, base and collector electrodes, means to apply said regulated voltage to said emitter electrode, means including a resistive voltage divider having first and second resistors for applying said standard voltage to said base electrode, means for applying a signal from said source of alternating voltage as an additive component to said standard voltage at said base electrode, means for deriving an output signal from said collector electrode, said solid state device being alternately conductive and cutoff, in operative mode and assuming a single state in inoperative mode, a fail-safe capacitor-diode rectifier circuit connected to the output of said solid state device, said output signal causing said fail-safe rectifier circuit to produce a substantial voltage signal level in said operative mode and to produce a negligible voltage signal level in said inoperative mode, and means for applying said voltage signal level to said amplification stage so that said amplification stage is enabled in response to said substantial voltage signal level and disabled in response to said negligible voltage, said monitoring means assuming said inoperative mode upon failure of any non-resistive component thereof.

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